1. Field of the Invention
This invention relates to a transistor device, and more particularly to improvements of the mount structure of a bi-polar transistor pellet.
2. Description of the Related Art
A conventional bi-polar transistor is structured as shown in FIGS. 1A and 1B. As shown, square plate 21 made of insulating material is fixed onto metal heat sink 20. Radiator plate 22 is further fixed on insulating square plate 21. The bottom surface of 23A of transistor pellet 23 is secured on the upper surface of radiator plate 22. The bottom surface of pellet 23 serves as a collector electrode. A narrow lead terminal continuously extending from radiator plate 22 is used as collector terminal 24. A couple of lead terminals, which are located on both sides of collector terminal 24, are used as emitter and base terminals 25 and 26, respectively. These terminals 25 and 26 are connected via metal lead wires 27 and 28 to emitter and base electrodes 29 and 30 of transistor pellet 23, respectively. Enclosure 31 hermetically encloses radiator plate 22, transistor pellet 23, lead terminals 24, 25 and 26, and metal lead wires 27 and 28. These lead terminals 24 to 26 are partially extended outside from the enclosure, as shown. Enclosure 31 is made of resin mold and i provided for protecting those elements from outside.
To use the transistor device in a grounded-base or common-base connection mode, base terminal 26 and heat sink 20 are electrically interconnected by means of an electrical conductive member (not shown). In this common-base connection mode, a feedback capacitance Cob between the collector and the base of the transistor device is the sum of capacitances Cob1 and Cob2. The capacitance Cob1 exists between the collector electrode and the base electrode, which are both on transistor pellet 23. The capacitance Cob2 exists between radiator plate (collector) 22 and heat sink (base) 20. The capacitance Cob1 between the collector and base electrodes on pellet 23 is approximately 4 pF, in the case of transistor whose breakdown voltage is 100 V or more, and saturation collector current is 200 mA. The capacitance Cob2 between radiator plate 22 and heat sink 20 depends on the size of radiator plate 22, and the material, thickness, and dielectric constant of insulating square plate 21. For example, the size of radiator plate 22 is 4.times.5 mm. The square plate 21 is made of boron nitride, 1 mm thick, and 8.1 in dielectric constant. The feedback capacitance Cob2 is approximately 14.33 pF. Hence, the capacitance Cob is 18.33 pF=4 pF+14.33 pF.
Some circuits often require use of the transistor devices with a small feedback capacitance Cob. A typical example of such circuits is a high frequency output circuit, e.g., an ultra-fine video signal output circuit. This circuit uses the transistor in a cascode fashion. In this circuit, to obtain the specified cutoff frequency, the feedback capacitance must be small, 5 pF or less. One of the conventional approaches to reduce the feedback capacitance Cob is to increase the thickness of insulating square plate 21. However, a figure of the feedback capacitance as attained by the approach fails to reach the 5 pF or less. For example, when thickness of square plate 21 is increased to 5 mm, the capacitance Cob2 between radiator plate 22 and heat sink 20 is approximately 2.8 pF, and the feedback capacitance Cob is approximately 6.8 pF at most. The approach involves an additional problem. Use of boron nitride for this insulating square plate 21 of 5 mm thick would increase the cost to manufacture.